Continue Experimentation with Pin Ergonomics

[nikomatsakis]

Metadata
Point of contact	@frank-king
Team(s)	compiler, lang
Goal document	2025h2/pin-ergonomics

Summary

Continue experimenting with and fleshing out the design and semantics for the pin ergonomics experiment.

Tasks and status

• Implementation (@frank-king)
• Standard reviews (compiler )
• Design meeting (lang )
• Author call for testing blog post (@frank-king)

[nikomatsakis]

This issue is intended for status updates only.

For general questions or comments, please contact the owner(s) directly.

[frank-king]

Status update:

Regarding the TODO list in the next 6 months, here is the current status:

Introduce &pin mut|const place borrowing syntax

• parsing: #135731, merged.
• lowering and borrowck: not started yet.

I've got some primitive ideas about borrowck, and I probably need to confirm with someone who is familiar with MIR/borrowck before starting to implement.

A pinned borrow consists two MIR statements:

1. a borrow statement that creates the mutable reference,
2. and an adt aggregate statement that put the mutable reference into the Pin struct.

I may have to add a new borrow kind so that pinned borrows can be recognized. Then traverse the dataflow graph to make sure that pinned places cannot been moved.

Pattern matching of &pin mut|const T types

In the past few months, I have struggled with the !Unpin stuffs (the original design sketch Alternative A), trying implementing it, refactoring, discussing on zulips, and was constantly confused; luckily, we have finally reached a new agreement of the Alternative B version.

• #139751 under review (reimplemented regarding Alternative B).

Support drop(&pin mut self) for structually pinned types

• adding a new Drop::pin_drop(&pin mut self) method: draft PR #144537

Supporting both Drop::drop(&mut self) and Drop::drop(&pin mut self) seems to introduce method-overloading to Rust, which I think might need some more general ways to handle (maybe by a rustc attribute?). So instead, I'd like to implemenent this via a new method Drop::pin_drop(&pin mut self) first.

Introduce &pin pat pattern syntax

Not started yet (I'd prefer doing that when pattern matching of &pin mut|const T types is ready).

Support &pin mut|const T -> &|&mut T coercion (requires T: Unpin of &pin mut T -> &mut T)

Not started yet. (It's quite independent, probably someone else can help with it)

Support auto borrowing of &pin mut|const place in method calls with &pin mut|const self receivers

Seems to be handled by Autoreborrow traits?